Image forming apparatus

ABSTRACT

An image forming apparatus includes: a main body; a unit withdrawably mounted in the main body; an exterior cover attached on a front face of the unit; a rail carrying the unit, the rail being withdrawable outwardly from the main body; a rail guide member for supporting the rail slidably; a locking mechanism for restricting movement of the unit and the rail; and a releasing mechanism for releasing the restriction of the locking mechanism. In the image forming apparatus, the exterior cover is operable to actuate the releasing mechanism and serve as a handle for the unit.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to image forming apparatuses such as a copyingmachine and a printer. More particularly, it relates to an image formingapparatus comprising a unit withdrawably mounted in an apparatus mainbody.

2. Description of the Related Art

There exists a known image forming apparatus in which a fixing unitincluding members for performing fixing of a toner image to a recordingsheet and is withdrawably mounted in an apparatus main body. In suchapparatus, a locking mechanism is provided so as to restrict a movementof the fixing unit at the time when such movement is not needed.However, in a conventional technology, an operation lever for unlockingthe locking mechanism provided in the fixing unit had been such a largepart extending in a longitudinal direction of the fixing unit thatspace-saving in the image forming apparatus was prevented.

In view of the above, a various kinds of locking mechanisms which arecompact and easy in operation for locking the fixing unit have beenproposed. For example, in a construction disclosed in JapaneseUnexamined Patent Publication No. HEI8-44131, a movable body which isconnected rotatably to a main body of an image forming apparatus by ahinge is provided, and a predetermined locking engaging member isprovided in the movable body on which the fixing device is mounted andin the main body of the image forming apparatus. Further, in JapaneseUnexamined Patent Publication No. 2003-241615, a fixing device includinga cover member serving as a handle gripped at the time of moving thefixing device and as an operating portion of the locking mechanism isdisclosed.

However, in the construction disclosed in the Japanese Unexamined PatentPublication No. HEI8-44131, strength of the exterior cover is likely tobe lowered since a part of the exterior cover is cut off and anoperation lever for the locking mechanism is provided therein.Consequently, it would be necessary to add strength separately, or thepart itself becomes larger and may prevent downsizing of the apparatus.

Further, in a construction disclosed in the Japanese Unexamined PatentPublication No. 2003-241615, a fixing cover is separately provided at aninner part of the exterior cover of the apparatus. Therefore, a user maybe required to withdraw the fixing device by gripping the fixing coverafter opening the exterior cover. It increases a number of operationsneeded for withdrawing the fixing device. Further, in the case of animage forming apparatus provided with a sheet conveyance passage betweenthe fixing unit and the exterior cover, the operating lever of thelocking mechanism prevents a conveyance of a recording sheet.Furthermore, there was a problem that a space between the exterior coverand the fixing unit for treating operation of sheet jamming becomesnarrow.

SUMMARY OF THE INVENTION

In view of the above-described problems, an object of the presentinvention is to provide an image forming apparatus including awithdrawing mechanism and a locking mechanism for a unit which saves aspace and is excellent in operability.

The image forming apparatus according to an embodiment of the presentinvention accomplishing the object comprises: a main body; a unitwithdrawably mounted in the main body; an exterior cover attached on afront face of the unit; a rail carrying the unit, the rail beingwithdrawable outwardly from the main body; a rail guide member forsupporting the rail slidably; a locking mechanism for restrictingmovement of the unit and the rail; and a releasing mechanism forreleasing the restriction of the locking mechanism, wherein the exteriorcover is operable to actuate the releasing mechanism and serve as ahandle for the unit.

According to the construction, the exterior cover which constitutes apart of an exterior surface of the apparatus main body serves as amember for actuating the releasing mechanism of the locking mechanism ofthe unit and as a handle portion for withdrawing the unit from the mainbody of the apparatus. Thus, it becomes easier to withdraw the unit, andit would not be necessary to provide a separate operating lever forreleasing the locking mechanism. Accordingly, it contributes to reducingof a number of parts and downsizing of the apparatus.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a front sectional view for describing an internal structure ofan image forming apparatus according to an embodiment of the invention.

FIG. 2 is a perspective view showing an example of a fixing unit.

FIG. 3 is a perspective view showing a construction of a vicinity of anexterior cover in a state where the fixing unit is mounted in.

FIG. 4 is a side view showing a vicinity of the exterior cover in thestate where the fixing unit is mounted in.

FIG. 5 is a perspective view showing a construction of a vicinity of theexterior cover in a state where the fixing unit is withdrawn.

FIG. 6 is a side view showing a vicinity of the exterior cover in thestate where the fixing unit is withdrawn.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, an embodiment of the invention is described with referenceto drawings.

FIG. 1 is a front sectional view for describing an internal structure ofan image forming apparatus 100 according to an embodiment of theinvention. In the present embodiment, a digital composite machine isdescribed as an example of the image forming apparatus 100.

The image forming apparatus includes in its apparatus main body 1 animage forming section 2 for transferring an image onto a recording sheet9, a sheet feeding mechanism 10 for storing a stack of the recordingsheets 9 and feeding the recording sheets 9 to the image forming section2, a fixing unit 15 for fixing an image onto the recording sheet 9 towhich a transfer of the image is executed in the image forming section 2and an image reading section 5 for reading out a document image.

The sheet feeding mechanism 10 is detachably mounted in the apparatusmain body 1 and includes a plurality of sheet feeding cassettes 10 a, 10b, 10 c for storing recording sheets 9 and a stack bypass (manualfeeding tray) 10 d provided in upper part of the sheet feeding cassettes10 a, 10 b, 10 c. The recording sheets 9 stored in the sheet feedingcassette 10 a, 10 b, 10 c are sent out by a sheet feeding roller 11 oneafter another and fed to the image forming section 2 through a sheetconveyance passage 12 and a pair of registration rollers 13.

The image forming section 2 executes transferring process to therecording sheets 9 based on image data read out in the image readingsection 5 or electrically transferred from a computer and the like. Theimage forming section 2 includes a photoconductive drum 4 mountedrotatably about a shaft (direction A indicated by an arrow in FIG. 1)extending in forward and backward directions (direction perpendicular toa sheet of FIG. 1) of the apparatus. Further, a charging unit 3, anexposing unit (laser scanning unit and the like) 8, a developing unit 7,a transferring roller 14 and a cleaning device 16 are providedsuccessively around the photoconductive drum 4.

On a peripheral surface of the photoconductive drum 4, an electrostaticlatent image and a toner image based on the electrostatic latent imageare formed. For the photoconductive drum 4, a drum on which a layer ofamorphous silicon is formed can be preferably used.

The charging unit 3 charges an electric charge on the peripheral surfaceof the photoconductive drum 4 which is rotated in a direction of thearrow A about the shaft. The charging unit 3 applies electric charge onthe peripheral surface of the photoconductive drum 4 by a corona charge.In place of the charging unit 3, the charging roller may be adopted.

The exposing unit 8 irradiates a laser light ray having a properstrength based on an image data to be formed to the peripheral surfaceof the photoconductive drum 4. The irradiated laser light ray erases anelectric charge on the photoconductive drum 4 so as to form theelectrostatic latent image on the peripheral surface of thephotoconductive drum 4.

The developing unit 7 feeds a developer (hereinafter, referred to astoner particles) to the peripheral surface of the photoconductive drum 4to thereby attach toner particles to portions of the peripheral surfaceon which the electrostatic latent image is formed. Accordingly, a tonerimage is formed on the peripheral surface of the photoconductive drum 4.The toner particles are supplied to the developing unit 7 from a tonercontainer 6.

The transferring roller 14 makes the positively charged toner imageformed on the peripheral surface of the photoconductive drum 4transferred to the recording sheet 9 sent to the peripheral surface ofthe photoconductive drum 4. To realize this operation, the transferringroller 14 applies negative electric charge having an inversed polaritywith respect to the electric charge of the toner image to the recordingsheet 9. Thus, the recording sheet 9 sent to an immediately downwardposition of the photoconductive drum 4 is negatively charge while beingpressingly nipped by the transferring roller 14 and the photoconductivedrum 4, and then the toner image on the positively charged peripheralsurface of the photoconductive drum 4 is peeled off toward the surfaceof the recording sheet 9. Accordingly, the transferring process of thetoner image to the recording sheet 9 is performed.

The cleaning device 16 cleans up toner particles or other attachedobject resided on the peripheral surface of the photoconductive drum 4after the transferring process. In the cleaning device 16, a cleaningroller which comes in contact with the peripheral surface of thephotoconductive drum 4 is provided. By a rotation of the cleaningroller, the resided toner particles and objects attached on thephotoconductive drum 4 are cleaned. A portion of the peripheral surfaceof the photoconductive drum 4 cleaned by the cleaning device 16 moves tothe charging unit 3 again for the next image forming process.

The fixing unit 15 is adapted for fixing the toner image transferred tothe recording sheet 9 in the image forming section 2 by heat. The fixingunit 15 includes a fixing roller for applying heat to the recordingsheet 9 and a pressing roller facing the fixing roller. After thetransferring process, the recording sheet 9 is sent to a nip portionformed between the fixing roller and the pressing roller. By passingthrough the nip portion, the recording sheet 9 receives heat from thefixing roller so that the fixing process is executed thereto.

After passing through the fixing unit 15, a conveyance direction of therecording sheet 9 is directed to either one of the conveyance passagesbranched off in a plurality of directions by a branching member 17. Inthe case of forming the image only on one side of the recording sheet 9,the recording sheet 9 passes through a pair of side discharging rollers18 and is discharged onto a sheet discharging tray 19 in a state wherethe image surface of the recording sheet 9 is kept facing upward (faceup). Or, the recording sheet 9 is conveyed to a sheet conveyance passage20 extending in upward direction by the branching member 17 anddischarged to the intermediate discharging tray 23 through a pair ofintermediate discharging rollers 22 in a state where the image surfaceof the recording sheet 9 is kept facing downward (face-down). The sheetconveyance passage 20, the pair of conveyance rollers 21 and the pair ofintermediate discharging rollers 22 serves also as a switchback devicefor switching a conveyance direction of the recording sheet 9.

On the other hand, in the case of forming images on both sides of therecording sheet 9, after passing through the fixing unit 15, therecording sheet 9 is conveyed to the sheet conveyance passage 20 by thebranching member 17. Then, after a rear end of the recording sheetpasses through the branching member 17, the pair of conveyance rollers21 and the pair of intermediate discharging rollers 22 are rotatedreversely so that the conveyance direction is switched. Thereafter, therecording sheet 9 is conveyed to a double-face printing passage 24 bythe branching member 17 and conveyed to the image forming section 2again in the state where a side of the recording sheet 9 not beingformed an image faces upward. On the recording sheet 9 conveyed again tothe image forming section 2, the next image is formed. Then, therecording sheet 9 is conveyed to the fixing unit 15, and the imageformed on the recording sheet 9 is fixed. Thereafter, the recordingsheet 9 is discharged to outside of the apparatus by the pair of sidedischarging rollers 18 or by the pair of intermediate dischargingrollers 22.

In such image forming apparatus 100 as described above, the fixing unit15 is mounted in the apparatus main body 1 withdrawably in a directionparallel to the sheet conveyance direction. Consequently, even in thecase where the recording sheet 9 is jammed in a vicinity of the fixingunit 15, the recording sheet 9 can be removed easily.

FIG. 2 is a perspective view showing the fixing unit 15 withdrawablymounted. As shown in FIG. 2, the main body of the fixing unit 15(including the fixing roller, the pressing roller and the like) isplaced on two rails 30 a, 30 b. The rails 30 a, 30 b a reslidable in ahorizontal direction along an unillustrated rail guide provided in oneside of the apparatus main body 1 (directions of arrow A-A′). The fixingunit 15 is provided with a fixing input gear 31 and driving gears 32,33, 34 as a driving system. The driving gears 32, 33, 34 are adapted fortransmitting a driving force generated by an unillustrated fixingdriving motor (driving means) to the fixing input gear 31.

In the case where a sheet jamming occurs in a vicinity of a mountingportion of the fixing unit 15, the fixing unit 15 is withdrawn togetherwith the rails 30 a, 30 b by a user in the direction A from theapparatus main body 1. Accordingly, the fixing input gear 31 is detachedfrom the gear 32 so that driving of the fixing unit 15 is stopped. Then,after the jamming is treated and the fixing unit 15 is inserted towardthe direction of the arrow A′, the fixing input gear 31 is engaged withthe gear 32.

Next, a withdrawing mechanism and a locking mechanism of the fixing unit15 are specifically described. FIG. 3 is a perspective view in a viewpoint from inner part of the apparatus showing a construction of avicinity of an exterior cover in the state where the fixing unit 15 ismounted in. FIG. 4 is a side view showing a vicinity of the exteriorcover in the state where the fixing unit 15 is mounted in. Forconvenience in description, a main body of the fixing unit 15 (refer toFIG. 2) is not illustrated in FIGS. 3 and 4.

In addition to the main body and the rails 30 a, 30 b shown in FIG. 2,the fixing unit 15 includes a conveyance guide member 35. Further, anexterior cover 40 constituting a part of the external surface of theapparatus main body 1 is attached on an outer side of the conveyanceguide member 35 (a front face of the fixing unit 15).

The conveyance guide member 35 is a guide member (described as abranching member 17 in FIG. 1) for guiding the recording sheet 9 whichhas passed through a fixing roller of the fixing unit 15 to the sheetconveyance passage 20, the opposite sides printing passage 24 (refer toFIG. 1) or a side discharging portion 25 provided in an exterior cover40. The conveyance guide member 35 is fixed on leading edges of the tworails 30 a, 30 b so as to bridge the rails.

On the conveyance guide member 35, a side discharging roller 18 a isprovided. The side discharging roller 18 a is rotated in a sheetdischarging direction by an unillustrated driving means. A pair of sidedischarging rollers 18 are constructed by the side discharging roller 18a and a discharging roller 18 b. The discharging roller 18 b pressinglycomes in contact with the side discharging roller 18 a provided in theexterior cover 40 and is rotationally driven. On a sheet conveyancesurface 35 a of the conveyance guide member 35, a guide roller 41 forsmoothly conveying the recording sheet is provided.

The exterior cover 40 does not only constitutes a mere part of anexterior surface of the apparatus main body 1 but also serves as ahandle used in a case of withdrawing the fixing unit 15 from theapparatus main body 1 or in a case of mounting the fixing unit 15 in theapparatus main body 1.

On upper portions of opposite ends of the exterior cover 40, a pair of ahooking member 38 projects toward inner part of the apparatus main body1. The hooking member 38 and an engaging pin 42 provided on a side ofthe apparatus main body 1 constitute the locking mechanism. Namely, thehooking member 38 engages with the engaging pin 42 so that a movement ofthe fixing unit 15 and the rails 30 a, 30 b in a horizontal direction isrestricted.

The exterior cover 40 is rotatably attached to the conveyance guidemember 35 with a rotational movement supporting point 39 projecting frominner surface of the hooking member 38. Accordingly, the exterior cover40 is rotated in directions of arrows B-B′ together with the hookingmember 38. In other words, the exterior cover 40 is shiftable over avertical posture (a first posture as referred to FIGS. 3 and 4) and arotated posture (a second posture as referred to FIGS. 5 and 6). In thevertical posture, an entirety of the exterior cover 40 comes close tothe conveyance guide member 35 (the fixing unit 15) so that the exteriorcover 40 is not rotated substantially. In the rotational posture, theexterior cover 40 is rotated a predetermined angle about a rotationalmovement supporting point 39 from the vertical posture so that a lowerportion of the exterior cover 40 is separated from the conveyance guidemember 35. The exterior cover 40 serves also as a releasing lever(releasing mechanism) for releasing the engagement of the hooking member38 and the engaging pint 42 by its rotation. Further, the exterior cover40 is biased in a direction of the arrow B′ by a torsion spring 43provided in the rotational movement supporting point 39.

FIG. 5 is a perspective view in a view point from an inner part of theapparatus showing a construction of a vicinity of the exterior cover 40in the state where the fixing unit 15 is withdrawn. FIG. 6 is a sideview showing the state same as that of FIG. 5. A withdrawing mechanismof the fixing unit 15 is specifically described with reference to theseFigures. At first, if the user grasps the lower end portion of theexterior cover 40 to withdraw the fixing unit 15, the exterior cover 40is rotated a predetermined angle in a direction of the arrow B about therotational movement supporting point 39 while resisting against thebiasing force generated by the torsion spring 43.

With the rotational movement of the exterior cover 40, the hookingmember 38 is also rotated a predetermined angle in a direction of thearrow B. Accordingly, as shown in FIG. 6, the engagement of the hookingmember 38 and the engaging pin 42 is released. Consequently, the fixingunit 15 and the rails 30 a, 30 b become movable in a horizontaldirection. Thus, by grasping the exterior cover 40 and withdrawing thesame in a direction of the arrow A, the fixing unit 15 is withdrawn withthe rails 30 a, 30 b outwardly from the apparatus main body 1.

Namely, the exterior cover 40 is so constructed as to serve as a handlefor withdrawing the fixing unit 15 and as a releasing lever for thelocking mechanism. Accordingly, components of the locking mechanism canbe reduced and placed in a space-conserving manner. Further, since it isnot necessary to form notches and the like in the exterior cover 40,strength and the design of the exterior cover 40 is improved.Furthermore, since the locking mechanism can be released while theexterior cover 40 is grasped, operability of the fixing unit 15 at thetime of withdrawing the exterior cover 40 is improved.

Since the fixing unit 15 is withdrawn together with the rails 30 a, 30b, the user can easily treat jamming occurred in a vicinity of thefixing unit 15 and replace the fixing unit 15. Further, in the casewhere the double-face printing passage 24 is provided between theconveyance guide member 35 and the exterior cover 40 (an arrow C in FIG.6), the user can easily treat jamming occurred in a vicinity of thedouble-face printing passage 24 by rotating the exterior cover 40 in adirection of the arrow B.

If the fixing unit 15 is inserted toward the inner part of the apparatusmain body 1, at first, an inclined surface 38 a of a leading end of thehooking member 38 runs upon the engaging pin 42 and then the rotates ina direction of the arrow B. Then, when the fixing unit 15 is mounted ata predetermined position, the inclined surface 38 a gets over theengaging pin 42. Thus, the hooking member 38 is rotated in the directionof the arrow B′ by the torsion spring 43 and is engaged with theengaging pin 42. Accordingly, the locking mechanism can be actuated byonly pressing the exterior cover 40 in a direction of the arrow A′ sothat operability at the time of inserting the fixing unit 15 isimproved.

Further, as long as it is so constructed that the hooking member 38 isrotated in accordance with a rotation of the exterior cover 40 in thesame direction, it is not necessarily the case that the rotationalmovement supporting point of the exterior cover 40 and the hookingmember 38 are made the same. For example, the hooking member 38 may beprovided in a side of a rotational edge of the exterior cover 40. Insuch a case, the engaging pin 42 on the side of the apparatus main body1 is placed in accordance with a position of the hooking member 38.

Furthermore, the rotational movement supporting point 39 of the exteriorcover 40 and of the hooking member 38 is provided on an upper end of theexterior cover 40 herein so that the fixing unit 15 is withdrawn bygrasping a lower end of the exterior cover 40. In place of thisconstruction, the rotational movement supporting point 39 can beprovided at a lower end of the exterior cover 40 so as to make the upperend potion a handle. In such a case, the hooking member 38 may bepositioned so that opening faces downward. Accordingly, engagement ofthe hooking member 38 and the engaging pin 42 is released by a rotationof the upper end of the exterior cover 40.

Furthermore, there is no limit on a rotational angle of the exteriorcover 40 as long as the angle is larger than the value at which theengagement between the hooking member 38 and the engaging pin 42 isreleased. However, if the rotational angle is too large, operability atthe time of withdrawing the fixing unit 15 by grasping the exteriorcover 40 gets worse. Thus, it is preferable that the angle is set to bea minimum value at which the engagement between the hooking member 38and the engaging pin 42 can be released.

The present invention is not limited to the embodiments described above,and other various modifications can be made within an area not departingfrom a purpose of the invention. For example, in the embodiment, theimage forming apparatus 100 having a construction of withdrawing thefixing unit 15 horizontally in the sheet conveyance direction isdescribed. However, the unit which is withdrawn outwardly from theapparatus is not limited to the fixing unit 15. For example, it may beother unit such as a transferring unit including a transferring roller14. Further, it may be so constructed that the fixing unit 15 iswithdrawn together with other units.

Further, in the above-described embodiment, the case where the exteriorcover 40 is rotatably attached to the conveyance guide member 35 and isbiased by the torsion spring 43 is described. However, other biasingmember such as a coil spring and a plate spring may be used in place ofthe torsion spring 43. In the case of making the hooking member 38engaged with the engaging pin 42 manually at the time of inserting unit,it may be so constructed that a biasing means is not provided. There isno special limitation on a mounting position of the exterior cover 40 aslong as the member is fixed on the rails 30 a, 30 b and is withdrawntogether with the rails 30 a, 30 b. The exterior cover 40 may beattached to a member other than the conveyance guide member 35.

Further, the present invention can be applied not only to the digitalcomposite machine but also to an intermediate transferring color imageforming apparatus in which toner images of respective colors aretransferred on a recording sheet at one time after the color images aresuccessively superimposed on a transferring belt. Furthermore, thepresent invention can be certainly applied to other types of imagingforming apparatuses such as an analog copying machine, a facsimilemachine and a laser printer. In the intermediate transferring colorimage forming apparatus, it may be so constructed that the intermediatetransferring roller is withdrawn in a sheet conveyance direction solelyor with the fixing unit 15.

As described above, the image forming apparatus according to the presentinvention includes: a main body; a unit withdrawably mounted in the mainbody; an exterior cover attached on a front face of the unit; a railcarrying the unit, the rail being withdrawable outwardly from the mainbody; a rail guide member for supporting the rail slidably; a lockingmechanism for restricting movement of the unit and the rail; and areleasing mechanism for releasing the restriction of the lockingmechanism, wherein the exterior cover is operable to actuate thereleasing mechanism and serves as a handle for the unit.

Accordingly, an image forming apparatus in which a withdrawing operationof the unit can be operated without operating an operation lever and thelike. Further, there is no necessity to provide an operation lever andthe like for releasing the restriction of the locking mechanism.Consequently, number of components of the locking mechanism can bereduced and a space can be saved so that the image forming apparatusbecomes compact and good in design.

In the above described construction, it is preferable that: the exteriorcover is shiftable over a first posture and a second posture differentfrom the first posture against the unit; and the locking mechanismincludes a hooking member projecting on an inner surface of the exteriorcover and an engagement member provided in the main body, the engagementmember being engageable with the hooking member when the exterior coveris in the first posture and disengeagable from the engagement memberwhen the exterior cover is in the second posture.

According to this construction, engagement and disengagement of thehooking member with respect to the engagement member can be performed bychanging the posture of the exterior cover.

Accordingly, an image forming apparatus having an exterior cover servingas a handle and an operation lever of a locking mechanism can berealized with a simple construction.

In this case, it is preferable that the exterior cover is supportedrotatably against the unit and that the exterior cover is not rotatedagainst the unit substantially in the first posture and is rotated apredetermined angle against the unit in the second posture.

According to this construction, the changing of the posture can beeasily made by rotating the exterior cover so that an operability can beimproved.

Furthermore, it is preferable that a biasing member which urges theexterior cover in such a direction that the hooking member engages withthe engagement member. Accordingly, the locking mechanism can beautomatically actuated at the time of inserting the unit so that anoperability of insertion is also improved.

Further, if a fixing unit is made the withdrawable unit, a withdrawingor inserting operation of the fixing unit can be made more easily at thetime of treating jamming occurred in vicinity of the fixing unit or atthe time of replacing the fixing unit.

This application is based on patent application No. 2005-220016 filed inJapan, the contents of which are hereby incorporated by references.

As this invention may be embodied in several forms without departingfrom the spirit of essential characteristics thereof, the presentembodiment is therefore illustrative and not restrictive, since thescope of the invention is defined by the appended claims rather than bythe description preceding them, and all changes that fall within metesand bounds of the claims, or equivalence of such metes and bounds aretherefore intended to embraced by the claims.

1. An image forming apparatus comprising: a main body; a unitwithdrawably mounted in the main body; an exterior cover attached on afront face of the unit; a rail carrying the unit, the rail beingwithdrawable outwardly from the main body; a rail guide member forsupporting the rail slidably; a locking mechanism for restrictingmovement of the unit and the rail; and a releasing mechanism forreleasing the restriction of the locking mechanism, wherein the exteriorcover is operable to actuate the releasing mechanism and serve as ahandle for the unit.
 2. An image forming apparatus according to claim 1,wherein: the exterior cover is shiftable over a first posture and asecond posture different from the first posture against the unit; andthe locking mechanism includes: a hooking member projecting on an innersurface of the exterior cover; and an engagement member provided in themain body, the engagement member being engageable with the hookingmember when the exterior cover is in the first posture and disengageablefrom the engagement member when the exterior cover is in the secondposture.
 3. An image forming apparatus according to claim 2, wherein:the exterior cover is supported rotatably with respect to the unit; thefirst posture is a state where the exterior cover is before rotatingwith respect to the unit, and the second posture is a state where theexterior cover is after rotating a predetermined angle with respect tothe unit.
 4. An image forming apparatus according to claim 2, furthercomprising a biasing member which urges the exterior cover in such adirection that the hooking member engages with the engagement member. 5.An image forming apparatus according to claim 1, wherein the unit is afixing unit.